It is well known that moving objects in digital video displayed on a hold-type display device such as a liquid crystal display (LCD) can appear blurry to an observer. The perceived blur is known to be caused in part by the relatively slow LC response of the liquid crystal cells. When compared with an impulse-type device such as a cathode ray tube (CRT) device, for example, an LCD device has a much slower brightness transition response time. The perceived blur is also caused in part by prolonged light emission inherent in the sample-and-hold driving technique commonly employed by LCD devices, which results in formation of after-images on the human retina. These after-images produce a blurred visual perception as the video sequence is being observed.
It has been proposed to compensate for the relatively slow response time of an LCD device by modifying the LCD device such that it adjusts the driving voltage applied to each device pixel by an amount proportional to a gray level transition. This is commonly known as the “overdrive” technique, and is described by H. Okurama, M. Akiyama, K. Takotoh, and Y. Uematsu in the publication entitled “A New Low Image-Lag Drive Method For Large-Size LCTVs,” (SID '02 Digest, pp. 1284-1287, 2002).
It has also been proposed to compensate for the prolonged light emission inherent in the sample-and-hold driving technique by modifying the LCD device using black frame insertion, as described by T. Kurita in the publication entitled “Consideration on Perceived MTF of Hold Type Display For Moving Images,” (IDW '98 Digest, pp. 823-826, 1998). U.S. Patent Application Publication No. 2003/0122772 to Park et al. also relates to black frame insertion and discloses a method for operating an LCD device by adjusting signals to the LCD device in order to control the ratio of real data to inserted black data. The ratio is controlled in accordance with the speed of the motion in the video stream. In particular, an increased ratio of black data to video data is said to prevent the appearance of motion blur.
Another technique used to compensate for prolonged light emission involves backlight blinking, as described by T. Furuhashi, K. Kawabe, J. Hirikata, Y Tanaka and T. Sato in the publication entitled “High Quality TFT-LCD System For Moving Picture” (SID '02 Digest, pp. 1284-1287, 2002).
In order to avoid modifying the hold-type display itself, it is known to apply video pre-processing to frames of the digital video sequence prior to display on the hold-type device in order to compensate for motion blur. For example, as described by K. Sekiya and H. Nakamura, in the publication entitled “Eye-Trace Integration Effect on the Perception of Moving Pictures and a New Possibility For Reducing Blur” (SID '02 Digest, pp. 930-933, 2002) the low-pass filtering effect of LCDs on a step function is analyzed and used to introduce waveform modulation in order to reduce motion blur in rectangular waveforms.
Another video pre-processing technique for motion blur compensation is detailed by M. Klompenhouwer and L. J. Velthoven, in the publication entitled “Motion Blur Reduction for Liquid Crystal Displays: Motion Compensated Inverse Filtering” (Visual Communications and Image Processing, pp. 690-698, 2004). According to this technique, blur is reduced by processing each pixel with a one-dimensional high-pass filter oriented in the local direction of motion. As with other known inverse filtering methods, however, noise can be problematic.
U.S. Patent Application Publication No. 2003/0006991 to De Haan et al. discloses a method for suppressing motion blur due to motion integration along a trajectory that employs inverse integration filtering of the video signal. High spatial frequency properties of the video signal are determined such that filtering in flat regions of the image can be avoided. The method is purported to reduce the noise modulation that is typical of other inverse filtering methods.
Other pre-processing techniques have also been considered. For example, U.S. Patent Application Publication No. 2002/0154088 to Nishimura discloses a method for displaying an image on an LCD display which, based on a determined motion vector, switches between an image signal making up the input image and a blanking signal. The method is purported to reduce the flickering, trailing and image-retention phenomenon caused by prior attempts to compensate for the physical characteristics of an LCD display compared to a CRT device. The method factors the speed of image motion to alter the rate at which the non-image signal is displayed. In particular, when motion of an image is fast, the rate at which the non-image signal is displayed during one frame is increased and control is exerted such that the level of the non-image signal comes nearer to the level of white color rather than the level of black color. The converse is true when the motion of an image is slow. As such, display luminance and contrast is generally increased.
It is an object to provide a novel method and system for estimating motion and compensating for perceived motion blur in digital video.